When a sample of fluid is placed on a solid substrate, the leading edge of the sample meets the solid surface at a precise angle, which is referred to as the contact angle. Technically, the contact angle is the angle between the liquid/solid interface and the liquid/vapor interface at the point of contact. The more exactly the contact angle can be measured, the more precisely the wetting properties of the sample fluid can be determined.
In current practice the contact angle is measured directly from the profile of a liquid droplet which is placed on a solid substrate. In the present state of the art, precise measurement of the contact angle is difficult because the angle between the solid surface and the tangent line at the contact point (FIG. 1) is subject to measurement errors. Any local roughness or surface contamination affects the angle. Furthermore, the measurement gives a different result for advancing or receding drops.
Various patents and other teachings address the measurement of contact angle between fluids and solids; most, if not all, tend to measure the angle directly by placing a droplet on the surface and then use some way to measure the angle. See, for instance, U.S. Pat. No. 5,268,733 (Issued Dec. 7, 1993) for “Method and Apparatus for Measuring Contact Angles of Liquid Droplets on Substrate Surfaces”; U.S. Pat. No. 5,080,484 (Issued Jan. 14, 1992) for “Method of Measuring the Contact Angle of Wetting Liquid on a Solid Surface”; U.S. Pat. No. 4,688,938 (issued Aug. 25, 1987) for “Method and Apparatus for Determining the Contact Angle of a Drop of Liquid Placed on Solid or Liquid Horizontal Substrate”; and U.S. Pat. No. 4,050,822 (issued Sep. 27, 1977) “Drop Measuring Apparatus, and a Method of Evaluating Materials Wettability”.
The first contact angle measuring device, known as a goniometer, was designed by Dr. William Zisman of the Naval Research Laboratory in the early 1960's. The original contact angle goniometer used a special microscope to measure the contact angle of a drop from its profile. Modern instruments use digital cameras and software to capture and analyze the drop profile. Commercial goniometers have been made available for the last forty years, e.g., by Ramé-Hart Instrument Company, New Jersey, U.S.A., (http://www.rameheart.com/).